Neat alcohols as alternative fuels for Diesel engines have been under investigation for several decades. In one attempt, commercial engine development was addressed in the early 1980's with the production of a methanol-fueled, compression-ignited (CI) engine. The combination of low cetane number and large enthalpy of vaporization was addressed by retaining hot combustion products within the cylinder to create sufficient ignition conditions near top dead center (TDC) with a relatively high compression ratio, up to 23:1. The engine operated in a direct-injection, two-stroke fashion, and the amount of exhaust gas removed during the scavenging process was controlled for stable combustion.
Glow plugs were used for cold-start operation only. Particulate measurements from the engine were non-negligible, and this was attributed in large part to burned engine oil since the particulate matter (PM) emissions did not increase with fuel to air ratio.
In another attempt, testing was conducted with a single-cylinder, four-stroke engine operating on neat methanol, as well as wet methanol and methanol blended with higher alcohols. In order to achieve consistent autoignition, a compression ratio of 18.9:1 with intake air preheating to 138° C. was required, which was interpreted as the simulation of turbocharging without intercooling. The reported smoke data is very low across the load range tested, which remained below stoichiometric conditions.
A low heat rejection (LHR) engine has been used to investigate emissions and performance for direct-injection, spark-assisted combustion of methanol and ethanol as compared to Diesel fuel. In this study, a single-cylinder engine with a compression ratio of 16.5:1 was outfit with an insulated piston face and head. The measurements of Bosch smoke number were reported being essentially zero for both alcohols, as compared to the Diesel-fuel baseline. The apparent heat release rate data indicated that the increase in compressed air temperature due to LHR surfaces allowed combustion to occur in two phases: that initiated by the spark plug, and that attributed to autoignition.
Ethanol has been researched in a partially premixed combustion (PPC) approach. Here, a significant amount of fuel-air mixing is desired before combustion, although a separate late injection is used for phasing control. The tested equivalence ratios approached, but did not meet, stoichiometric conditions. Peak temperatures were limited by the overall lean combustion and the use of exhaust gas recirculation (EGR) for nitrogen oxides (NOx) avoidance. Measurements from the PPC split injection strategy show low soot emissions, as expected from the large amount of pre-mixing. In this attempt, intake boosting and intake air preheating were required for ignition.
The inherent nature of low soot emissions from oxygenated fuels was previously analyzed in detail. It was shown experimentally that if the oxygen mass fraction of a fuel is greater than 30%, the measured smoke number is nearly zero. This may be achieved through the use of either neat or blended fuels. From this perspective, it is clear that both methanol (50% oxygen mass fraction) and ethanol (35% oxygen mass fraction) are attractive choices, and are expected to emit low levels of soot. Similarly, dimethyl ether (DME) has the same atomic composition as ethanol and is also expected to form little soot.
The intermolecular bonding of DME is different than ethanol, with the oxygen atom located between two methyl groups. As a result, its properties are different. Most notably, it has a Diesel-fuel-like cetane rating, in the range of 55-60. Its early use in Diesel engines was investigated as an ignition improver for DI methanol in a compression ignition engine. The DME was admitted into the intake air, and the direct injection of methanol was sufficiently ignited. It was believed that the lean DME-air mixture combusted during the compression stroke, thus raising the ambient gas temperature to a level that was acceptable for methanol ignition delay. As a neat DI fuel, DME has been tested and shown to enable clean and efficient operation of a Diesel engine. It is noteworthy that DME requires storage at several bar above atmospheric pressure in order to remain a liquid.
The use of neat alcohols, namely methanol and ethanol, in direct-injection, compression-ignited engines is difficult, most notably due to their poor ignitability. What is needed is a high temperature combustion strategy that enables the used of oxygenated and inherently low-sooting fuels for heavy-load applications.